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BA-1015: Bulk Water Control Methods for Foundations

Abstract:

Successfully executing strategies to control bulk water for foundations is critical for building durability, indoor air quality, and creating acceptable conditions and/or living spaces within the foundation space. Although the energy impacts of properly done bulk water control are small to insignificant, it should be considered a base requirement for any high performance house. In addition, measures such as basement insulation are predicated on properly managed foundation bulk water.

Executive Summary

Successfully executing strategies to control bulk water for foundations is critical for building durability, indoor air quality, and creating acceptable conditions and/or living spaces within the foundation space. Although the energy impacts of properly done bulk water control are small to insignificant, it should be considered a base requirement for any high performance house. In addition, measures such as basement insulation are predicated on properly managed foundation bulk water.

The fundamental concepts that must be understood at the planning phase (or during the initial inspection of an existing home) are the following:

Surface water must be managed: the impermeable roof surfaces concentrate rainfall at points around the perimeter of the building; this water must be shed off and away from the foundation. This is typically done with gutters/eavestroughs and downspouts that are directed away from the foundation (or run to a storm sewer, if permitted), correctly grading the site away from the foundation, and reducing the water permeability of the surface around the foundation. The goal is to saturate or "load" the soils around the foundation with as little additional water as possible.

Ground water must be managed: in order to prevent water entry into the foundation, it is necessary to prevent water accumulation against the foundation walls and/or under the slab (or ground cover). Water accumulation results in hydrostatic head pressure, which will push water through any available joints, imperfections, or cracks in the foundation. Accumulation is prevented by the use of drainage: measures include the use of free draining backfill and/or drainage board around the foundation wall, a functional footing drain directed to daylight (a downhill location) or a sump, and drainage via granular fill below the slab.

In terms of system interactions, improvement of bulk water control in foundations can only improve conditions for other building systems. For instance, control of roof drainage and directing it away from the building will reduce the extent of splashback onto exterior walls, thus improving durability and reducing the risk of aesthetics problems.

In terms of evaluating cost-value comparisons, it must be noted that repair of a failed foundation bulk water control system is time consuming, disruptive to occupants, destructive to exterior landscaping, and very expensive. It is likely that it is more cost-effective to specify foundation bulk water control details during the construction phase, rather than retrofitting measures to foundations showing bulk water problems.

Bulk water control measures are typically retrofitted to existing foundations when water control issues (and complaints) need to be solved, when the basement space is being renovated into conditioned space, and/or when interior insulation is being installed.

The above grade measures described previously apply to retrofit situations; in fact, many problem cases can be solved with grading and surface drainage. It is always better to intercept groundwater before it gets to a foundation wall. Exterior perimeter drainage is always preferable to interior perimeter drainage. However, in renovations, exterior perimeter drainage may not be present or may not be practical or possible. In such cases, interior perimeter drainage can be used and connected to an interior sump pump.

Another technique is to use an exterior impermeable material to minimize rain and groundwater entering below grade spaces, commonly known as an “apron,” “skirt,” or “ground roof.” This detail could be considered a “below grade overhang” for shedding water away from the foundation and preventing soil saturation. This method has the advantage of improving bulk water control of the foundation with minimal excavation (i.e., not down to the footings).

1 Home and/or Document Inspection

Successfully executing strategies to control bulk water for foundations is critical for building durability, indoor air quality, and creating acceptable conditions and/or living spaces within the foundation space. Although the energy impacts of properly done bulk water control are small to insignificant, it should be considered a base requirement for any high performance house. In addition, measures such as basement insulation are predicated on properly managed foundation bulk water.

The fundamental concepts that must be understood at the planning phase (or during the initial inspection of an existing home) are the following, and as demonstrated in Figure 1:

Figure 1: Foundation bulk water control overall concepts

Surface water must be managed: the impermeable roof surfaces concentrate rainfall at points around the perimeter of the building; this water must be shed off and away from the foundation. This is typically done with gutters/eavestroughs and downspouts that are directed away from the foundation (or run to a storm sewer), correctly grading the site away from the foundation, and reducing the water permeability of the surface around the foundation. The goal is to saturate or “load” the soils around the foundation with as little additional water as possible.

Ground water must be managed: in order to prevent water entry into the foundation, it is necessary to prevent water accumulation against the foundation walls and/or under the slab (or ground cover). Water accumulation results in hydrostatic head pressure, which will push water through any available joints, imperfections, or cracks in the foundation. Accumulation is prevented by the use of drainage: measures include the use of free draining backfill and/or drainage board around the foundation wall, a functional footing drain directed to daylight (a downhill location) or a sump, and drainage via granular fill below the slab.

The specific strategies that must be implemented (or inspected for) will be covered in more detail in Section 3, “Strategy Implementation Details.” But common signs of moisture problems in existing foundations can be manifested as visible water or puddles, staining of interior (or visible) finishes, mold growth, and efflorescence (water-borne white mineral salt deposits). Note that interior finishes or insulation may conceal the presence of bulk water issues for extended periods of time, and problems may not be evident until damage is extensive (as shown in Figure 4). Some guidance on diagnosing moisture issues is given by US EPA (2010) under “Moisture (Mold and Other Biologicals)” and US EPA (2009) (under “Moisture Control → Water Managed Site and Foundation”).

In addition, additional information can be gleaned by characterizing foundation bulk water issues by their frequency and/or time of occurrence. For instance, wetting events could be close to constant in nature (i.e., occurring throughout the year), or seasonal (e.g., only during spring/fall rains, or rising groundwater conditions). In addition, physical the location can provide some indication to the cause (close to an area of exterior wetted soil or disconnected downspout).

2 Tradeoffs

2.1 System Interaction

In general, improvement of bulk water control in foundations can only improve conditions for other building systems. For instance, control of roof drainage and directing it away from the building will reduce the extent of splashback onto exterior walls, thus improving durability and reducing the risk of aesthetics problems.

As discussed above, interior insulation or interior finishing of a basement is not recommended unless bulk water issues are brought under control.

One potential negative system interaction might be moisture-sensitive expansive soils: suddenly reducing the moisture content of the soil surrounding the foundation might result in shrinkage, ground movement, and foundation movement/cracking. These types of soils are known to occur in Texas, California, Virginia and Colorado.

2.2 Cost and Performance Tradeoffs

In general, cost decisions to execute (or not execute) foundation bulk water control details are not made on a quantitative basis. In contrast, energy upgrade measures can be quantified in terms of simple payback, energy return on investment, or other metrics. These metrics can be compared in terms of their relative value, providing a useful go/no-go measurement.

Failures of bulk water control measures will result in homeowner complaints, builder callbacks, durability issues, and potential indoor air quality issues, all of which would result in significant financial liabilities. Therefore, the decision to invest or not invest in a given measure is based on a risk assessment (either formal or informal), depending on soil conditions, site conditions, foundation type, and other factors. These conditions vary widely and can be affected by local and site variations. Some guidance is provided by various model codes. For instance, the International Residential Code (ICC 2009) allows for the elimination of foundation drainage under certain soil conditions (see §R405: Foundation Drainage). It must be realized, though, that any elimination of bulk water control measures increases risks of problems.

Overall, it must be noted that repair of a failed foundation bulk water control system is time consuming, disruptive to occupants, destructive to exterior landscaping, and very expensive. It might be most cost-effective to maximize bulk water control measures during construction of the foundation, as opposed to retrofits after problems occur. As an example of this type of comparison, an air gap membrane (a highly robust strategy easily installed during construction) has a material cost of roughly 50¢ per square foot. For a 1200 sf footprint house (30’ x 40’), this would be roughly $500 in materials. In comparison, excavation and retrofit of exterior basement drainage details might have costs in the range of several thousand to tens of thousands of dollars.

In existing houses, measures are typically taken when there is a complaint of bulk water intrusion into the foundation, or problems are found due to bulk water intrusion (e.g., basement mold issues). At that point, cost-performance trade offs are typically a smaller concern than solving the problem. Note that exterior retrofit measures that require excavation should be executed with appropriate safety measures. For instance, services must be located and identified, and OSHA approved methods should be used for excavation (OSHA 1999; see Section V: Chapter 2 Excavations: Hazard Recognition In Trenching and Shoring).

Critical Takeaways

In general, improvement of bulk water control in foundations can only improve conditions for other building systems.

Interior insulation or interior finishing of a basement is not recommended unless bulk water issues are brought under control.

It is likely that it is more cost-effective to specify foundation bulk water control details during the construction phase, rather than retrofitting measures to foundations showing bulk water problems.

3 Strategey Implementation Details

Rainwater, surface water and groundwater will wick through concrete and masonry materials. This can be a problem in two ways: building materials touching the foundation may grow mold, decay, corrode or dissolve; or the migrating water might evaporate into the basement or crawlspace and cause high humidity and/or condensation problems in the foundation and the upper part of the building. The fundamental principles of groundwater control are to keep rainwater away from the foundation wall perimeter and to drain groundwater with sub-grade perimeter drains before it gets to the foundation wall. This applies to basements, crawlspaces and slabs regardless of whether they are newly constructed or undergoing rehabilitation.

This section is broken down into several portions, starting with an explanation of above grade strategies in detail (including roof runoff and exterior grading), and then below grade strategies (including foundation wall drainage, footing drains, sump pit drainage, and capillarity through footings). This is followed by example building sections for basements, crawl spaces, and slabs on grade. The section concludes with a description of various available retrofit measures for addressing bulk water problems in existing foundations.

3.1 Above Grade Measures

Many foundation bulk water problems start with above grade drainage issues, and this should be the first place examined when diagnosing issues in existing buildings. The primary means of addressing these issues is with careful management of roof water runoff, and proper exterior grading.

3.1.1 Roof Runoff

Impermeable roof surfaces concentrate rainfall at points around the perimeter of the building; this water must be shed off and away from the foundation. Roof overhangs will help shed water away from the foundation, and also protect the above grade walls.

Figure 6: Directing roof runoff away from building using overhangs and grading

Part of the solution to keep water away from foundation is to use gutters/eavestroughs and downspouts that are directed away from the foundation (or run to a storm sewer, if permitted).